Siemens SIPROTEC 7SD610 Manual page 86

Functions
2.5 Restricted Earth Fault Protection (optional)
It was assumed in the above examples that the currents 3I
faults which is, is in fact, true for the primary measured quantities. Current transformer saturation may, however,
feign a phase displacement between the starpoint current and the sum of the phase currents which reduces
the restaint quantity. With ϕ(3I
direction determination using the method of summation and and difference comparison.
Figure 2-31
The restraint quantity can be influenced by a factor k. This factor has a certain relationship to the limit angle
.
ϕ
limit
This limit angle determines for which phase displacement between 3I
grows to ∞, i.e. no pickup occurs. In 7SD610 k = 4.
In the aforementioned example 1), the restraint quantity I
the tripping quantity I
The threshold angle ϕ
Figure 2-32 shows the operating characteristics of the restricted earth fault protection dependent of the phase
displacement between 3I
86
"; 3I ') = 90° the restraint quantity is zero. This corresponds to the conventional
0 0
Phasor diagram of the restraint quantity during external fault
.
REF
= 100°. That means no trip is possible for phase displacement ϕ(3I
Limit
" and 3I ' for a constant infeed ratio |3I
0 0
" and 3I ' are in counter-phase for external earth
0 0
" and 3I ' the pickup value for 3I
0 0
is quadrupled once more; thus, it becomes 8 times
Rest
"| = |3I '|.
0 0
C53000-G1176-C145-6, Release date 02.2011
" = 3I '
0 0
"; 3I ') ≥ 100°.
0 0
SIPROTEC, 7SD610, Manual